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The five Tunisians detained are suspected of being part of a network connected to Anis Amri, the Tunisian who killed 12 people when he drove a truck into a Christmas market in Berlin in December 2016, judicial sources told Reuters.

Amri was shot dead by Italian police near Milan four days after the Berlin attack.

Prosecutors suspect the arrested Tunisians of crimes including planning international terrorism, falsifying documents and helping illegal immigration of radicalized Tunisians, the sources said.

However, the sources said there was no evidence the suspects participated directly in the Berlin truck attack or that they were preparing any imminent attack in Italy.

As part of their operation, police were carrying out searches in Rome, the southern port city of Naples and the central and southern towns of Latina, Viterbo, Caserta and Matera, the sources said.

Interior Minister Marco Minniti said on Wednesday the security threat to Italy from Islamic State supporters was higher than ever because more foreign fighters were attempting to return to Europe via Italy after the jihadist group suffered defeats in Syria and Iraq.

So far this year, 29 foreigners, mostly Muslims, have been expelled from Italy as suspected threats to national security.